In view of environmental concerns, improvements to the fuel efficiency of means of transport such as cars, motorcycles, buses, and railcars have recently be considered as a critical issue. One actively-pursued approach to boosting fuel efficiency has been a reduction in vehicle body weight. The reduction in vehicle body weight relies heavily on lowering the weight of the materials used to fabricate the body components, specifically on reducing the thickness of sheet steels. However, the reduction in sheet material thickness brings about deteriorations of rigidity and collision safety performance.
Because the strength enhancement of the materials which are used for the components is an effective way to increase the collision safety, high-strength steel sheets having compositions of mild steels are utilized in automobile impact absorption components. However, mild steels are poor in corrosion resistance; and therefore, multi-painting is essential for their use. They cannot be used for unpainted or lightly painted components, and the multi-painting inevitably increases costs. Cr-containing stainless steels are far superior to the mild steels in corrosion resistance. Therefore, the Cr-containing stainless steels are expected to have the potential to reduce weight by lowering the corrosion margin and to eliminate the need for painting.
Further, with regard to the collision safety improvement, in the case where a material having high impact absorption capability is utilized for a component such as a vehicle frame, the component collapses and deforms when the vehicle crashes; and thereby, it is possible to absorb the crash impact by the component collapse deformation. As a result, it is possible to lessen the impact on passengers during the collision. In other words, considerable merits can be realized regarding fuel economy improvement, reduction in vehicle body weight, simplification of painting and safety enhancement.
Austenite stainless steel sheets with high ductility, excellent formability, and excellent corrosion resistance such as SUS301L and SUS304 are generally used in vehicle components which are required to have corrosion resistance, for example structural components of railcars.
Patent Document 1 discloses an austenite stainless steel having excellent impact-absorbing capability at a high strain rate, which is intended for use mainly in structural components and reinforcing materials for railcars and ordinary vehicles. This stainless steel contains 6 to 8% of Ni and has an austenite microstructure. In the stainless steel, a work-induced martensite phase is generated during a deformation; and thereby, high strength is achieved during the high-speed deformation.
However, since a relatively large amount of Ni is contained, high cost is not avoided. Furthermore, stress corrosion cracking or aging cracking may occur depending on the chemical compositions or usage environment. Therefore, this austenite stainless steel has not been always adequate for use as a general-purpose structure.
Martensite stainless steel sheets which are imparted with high strength by quenching (for example, SUS420) do not contain Ni or contain Ni at a lower content than that contained in an austenite stainless steel; and therefore, the martensite stainless steel sheets are advantageous in terms of costs. However, the martensite stainless steel sheets have problems such as markedly low ductility and markedly poor toughness at a welded portion (weld toughness). Since there are large numbers of welded structures in automobiles, buses, and railcars, their structural reliability is greatly impaired by poor weld toughness.
Ferrite stainless steel sheets (for example, SUS430) are also advantageous in terms of costs as compared to the austenite stainless steels. However, since the ferrite stainless steel sheets have low strength, the ferrite stainless steel sheets are not suitable for components where strength is required. Furthermore, since the ferrite stainless steel sheets have low impact absorption energy during the high-speed deformation, it has been impossible to improve the collision safety performance. That is, particularly with regard to high-strength stainless steels containing a ferrite phase as the parent phase, because dynamic deformation properties in a high strain rate region at the time of vehicular crash are little understood, it has been difficult to apply the stainless steels to impact-absorbing components.
Further, the martensite stainless steels and the ferrite stainless steels exhibit markedly low formability in terms of elongation as compared to the austenite stainless steels. Therefore, even when a strength enhancement is achieved by means of solid-solution strengthening or precipitation strengthening (grain dispersion strengthening), there has been a major problem in that the stainless steels could not be formed into structural components.
On the other hand, in Patent Document 2 (not published at the time of filing the present application), the present inventors have disclosed a technique relating to a stainless steel for structural components with excellent impact-absorbing properties in which a Ni content is reduced and which contains a ferrite phase as the parent phase and 5% or more of a martensite phase as a main secondary phase. This is an invention similar to the present invention. However, since the secondary phase is mainly a martensite phase, a strain-induced plasticity does not occur. Therefore, the workability (elongation and work-hardening properties) is markedly low, and there has been a problem associated with component formability.
Further, Patent Documents 3 and 4 disclose techniques relating to austenite-ferrite stainless steels having excellent formability. In these techniques, a volume fraction of the austenite phase and a phase distribution of the austenite phase are adjusted so as to transform the austenite phase into a work-induced martensite phase during deformation, that is, to generate a so-called strain-induced plasticity. Thereby, a high ductility is attained. However, in the case where a steel material is applied for a structural component, work-hardening properties are important in the forming of the component, and a strength and an impact absorption performance are also important for the structural component. The techniques of Patent Documents 3 and 4 have not been sufficient for such requirements.
[Patent Document 1] Japanese Patent Application, Publication No. 2002-20843
[Patent Document 2] Japanese Patent Application No. 2006-350723
[Patent Document 3] Japanese Patent Application, Publication No. 2006-169622
[Patent Document 4] Japanese Patent Application, Publication No. 2006-183129